Determine the extent of interindividual variability and racial differences in estrogen metabolism and plasma concentration in postmenopauasal women on estrogen replacement therapy. Also, determine the mechanism responsible for the variability in plasma estrogen concentrations by examining alterations in and the effects of manipulation of CYP3A and CYP1A2 activity by: concomitant drug therapy, dietary factors, and CYP3A racial differences in hepatic and gastrointestinal metabolism of estradiol. This study is currently on hold awaiting support award.